Question: $ D = \left[\begin{array}{rrr}1 & 0 & 0 \\ 2 & 1 & 2 \\ 2 & 2 & 0\end{array}\right]$ What is $ D^{-1}$ ?
Solution: $ = \left[\begin{array}{rrr}1 & 0 & 0 \\ -1 & 0 & \frac{1}{2} \\ -\frac{1}{2} & \frac{1}{2} & -\frac{1}{4}\end{array}\right]$